1. Field of the Invention
This invention relates to a method for preparing a dummy wafer, and more particularly, to a method for preparing a dummy wafer used in a chemical vapor deposition (CVD) process.
2. Description of Related Art
Chemical vapor deposition (CVD) is a common process used for forming a semiconductor thin film device that is theoretically based on controlling the reaction of gasiform reactant occurring with in a reacting furnace or tube. Generally, a CVD system basically consists of a reactor, gas-pumping system, an exhaust system, and a process control system.
For a tubular-type low-pressure chemical vapor deposition (LPCVD) system shown in FIG. 1, the most commonly used batch-type CVD system, the system contains a hot-wall reactor 10 made of annealed quartz. The reactor 10 is further surrounded by a three-zone heater 12 used for heating the reactor 10. While a LPCVD process is performed, the gasiform reactant is pumped into the reactor 10 from a door 14, and a boat 20 that carries a number of wafers 18 is then transported into the reactor 10 and placed at a pre-determined position. The exhaust gas of the LPCVD process is released by an exhaust system through a releasing end 16.
The boat 20 further contains a wafer stage for holding a certain number of wafers 18. When a CVD process is proceeded, it is required that the wafer stage is fully loaded in order to ensure the uniformity of the thickness of deposited materials. In case that a insufficient number of wafers 18 are placed on the wafer stage of the boat 20, the vacancies on the wafer stage need to be occupied with dummy wafers to ensure the uniformity of deposition.
Conventionally, black wafers are usually placed on the wafer stage of a CVD system to serve as dummy wafers while a CVD process is performed. Since the profile of a dummy wafer, which has a smooth surface, is very different from what of a productive wafer that has an accidented surface constructed by devices formed on the wafer, the amount of consumed reactant deposited on a dummy wafer is different from what of a productive wafer. Therefore, loading effect, that is, the thickness of material deposited on the surface of wafers is not uniform, occurs. The loading effect is more distinct when a CVD process of silicon nitride layer is performed.